In general, GaN based nitride semiconductor is applied to electronic devices that are high speed switching and high output devices such as blue and green light emitting diodes (LED), metal semiconductor field effect transistors (MESFET), and high electron mobility transistors (HEMT). In particular, the blue and green LEDs have already been produced and the global sales of the blue and green LEDs have exponentially increased.
The GaN based nitride semiconductor light emitting device is grown on a sapphire substrate or a SiC substrate. Then, an AlYGalYN polycrystalline thin film is grown on the sapphire substrate or the SiC substrate as a buffer layer at a low growth temperature. Then, an undoped GaN layer, a Si-doped n-GaN layer, or a mixture of the above two structures is grown on the buffer layer at a high temperature to form an n-GaN layer as a first electrode layer. Also, a Mg-doped p-GaN layer is formed at upper layer as a second electrode layer to manufacture a nitride semiconductor light emitting device. An emission layer (a multiple quantum well structure activation layer) is interposed between the n-GaN layer and the p-GaN layer.
In general, in undoped GaN nitride semiconductor to which impurities are not added, yellow emission peak is shown over a very wide region in a 550 nm wavelength bandwidth, which is caused by the defects of vacancy of Ga atoms (VGa) in GaN nitride semiconductor crystal growth.
Among such defects, when the n-GaN nitride semiconductor that is the Si-doped first electrode layer is grown, VGa is reduced so that very strong bandedge emission is shown. When the GaN nitride semiconductor is doped with silicon, VGa is exchanged by silicon. However, the dangling bond of N atoms continuously exists to affect the reliability of the light emitting device.
Also, the p-GaN layer that is the second electrode layer is formed by doping Mg atoms during the growth of crystal. The position of Ga is exchanged by Mg atoms implanted as a doping source during the growth of crystal to form the p-GaN layer. The Mg atoms are combined with a hydrogen gas separated from a carrier gas and a source to form Mg—H compound in the GaN crystal layer so that a high resistance body of about 10MΩ is obtained.
Therefore, after forming a pn conjunction light emitting device, a subsequent activation process of cutting off the Mg—H compound to exchanging the Mg atoms into the position of Ga is required. However, the amount of the light emitting device that operates as a carrier that contributes to emission in the activation process is 1017/cm3, which is much lower than Mg atomic concentration of no less than 1019/cm3 so that it is very difficult to form resistant contact.
Also, the Mg atoms that remain in p-GaN nitride semiconductor without being activated to a carrier operate as a center that traps the light emitted from an interface to rapidly reduce optical output.
In order to solve the problem, very thin transmissive resistant metal is used to reduce contact resistance so that current implantation efficiency is improved. In general, the optical transmittance of the thin transmissive resistant metal used in order to reduce the contact resistance is about 75 to 80% and the other operates as loss. Also, it has limitations on improving the optical output of the light emitting device during the growth of the crystal of the nitride semiconductor without improving the design of the light emitting device and the crystal properties of the emission layer and the p-GaN layer in order to improve internal quantum efficiency.